add workitems tests to improve code coverage (#243)

Improve WorkItems code coverage from;

```sh
----------------|---------|----------|---------|---------|------------------------------------------------------
File            | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s                                    
----------------|---------|----------|---------|---------|------------------------------------------------------
  workitems.ts  |   66.66 |    26.47 |   56.66 |   68.45 | 34-54,301,421,428-430,499-537,565-609,625-668
----------------|---------|----------|---------|---------|------------------------------------------------------
```

to

```sh
----------------|---------|----------|---------|---------|------------------------------------------------------
File            | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s                                    
----------------|---------|----------|---------|---------|------------------------------------------------------
  workitems.ts  |     100 |      100 |     100 |     100 |
----------------|---------|----------|---------|---------|------------------------------------------------------
```

## GitHub issue number

#57 (Don't close the issue, there are other tools to improve test
coverages)

## **Associated Risks**

None

## ✅ **PR Checklist**

- [x] **I have read the [contribution
guidelines](https://github.com/microsoft/azure-devops-mcp/blob/main/CONTRIBUTING.md)**
- [x] **I have read the [code of conduct
guidelines](https://github.com/microsoft/azure-devops-mcp/blob/main/CODE_OF_CONDUCT.md)**
- [x] Title of the pull request is clear and informative.
- [x] 👌 Code hygiene
- [x] 🔭 Telemetry added, updated, or N/A
- [x] 📄 Documentation added, updated, or N/A
- [x] 🛡️ Automated tests added, or N/A

## 🧪 **How did you test it?**

Run `npm run test` in the project root

Author: polatengin